U.S. patent number 9,309,369 [Application Number 14/168,830] was granted by the patent office on 2016-04-12 for polyimide aerogels with three-dimensional cross-linked structure.
This patent grant is currently assigned to The United States of America, as Represented by the Administrator of the National Aeronautics and Space Administration. The grantee listed for this patent is The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, The United States of America as represented by the Administrator of the National Aeronautics and Space Administration. Invention is credited to Mary Ann B. Meador.
United States Patent |
9,309,369 |
Meador |
April 12, 2016 |
Polyimide aerogels with three-dimensional cross-linked
structure
Abstract
A method for creating a three dimensional cross-linked polyimide
structure includes dissolving a diamine, a dianhydride, and a
triamine in a solvent, imidizing a polyamic acid gel by heating the
gel, extracting the gel in a second solvent, supercritically drying
the gel, and removing the solvent to create a polyimide
aerogel.
Inventors: |
Meador; Mary Ann B.
(Strongsville, OH) |
Applicant: |
Name |
City |
State |
Country |
Type |
The United States of America as represented by the Administrator of
the National Aeronautics and Space Administration |
Washington |
DC |
US |
|
|
Assignee: |
The United States of America, as
Represented by the Administrator of the National Aeronautics and
Space Administration (Washington, DC)
|
Family
ID: |
55643113 |
Appl.
No.: |
14/168,830 |
Filed: |
January 30, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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12571049 |
Sep 30, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08J
9/283 (20130101); C08J 2205/026 (20130101); C08J
2379/08 (20130101); C08J 2201/0502 (20130101); C08J
2205/024 (20130101) |
Current International
Class: |
C08J
9/28 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cooney; John
Attorney, Agent or Firm: Earp, III; Robert H.
Government Interests
ORIGIN OF THE INVENTION
The invention described herein was made by employees of the United
States Government and may be manufactured and used by or for the
Government for Government purposes without the payment of any
royalties thereon or therefore.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser.
No. 12/571,049 filed Sep. 30, 2009, pending.
Claims
I claim:
1. An aerogel consisting of a porous cross-linked polyimide network
comprising a plurality of anhydride end-capped polyamic acid
oligomers, wherein the oligomers (i) comprise oligomers having
repeating units of a dianhydride and a diamine that have terminal
anhydride groups, (ii) are cross-linked after the oligomers are
formed via a cross-linking agent, comprising three or more amine
groups, at a balanced stoichiometry of the amine groups to the
terminal anhydride groups, and (iv) have been imidized to yield the
porous cross-linked polyimide network.
2. The aerogel of claim 1, wherein the dianhydride is selected from
the group consisting of benzophenone-3,3',4,4'-tetracarboxylic
dianhydride, 2,2'-bis(3,4'-dicarboxyphenyl)hexafluoropropane
dianhydride, and biphenyl-3,3',4,4'-tetracarboxylic
dianhydride.
3. The aerogel of claim 1, wherein the diamine is selected from the
group consisting of 3,4-oxydianiline, 4,4'-oxydianiline,
p-phenylene diamine, 2,2'-dimethylbenzidine, bisaniline-p-xylidene,
4,4'-bis(4-aminophenoxy)biphenyl, and
2,2'-bis[4-(4-aminophenoxy)phenyl]propane.
4. The aerogel of claim 1, wherein the diamine comprises (i)
4,4'-oxydianiline and (ii) p-phenylene diamine or
2,2'-dimethylbenzidine.
5. The aerogel of claim 1, wherein the cross-linking agent is
selected from the group consisting of a triamine, an aliphatic
amine comprising three or more amines, an aliphatic triamine, an
aromatic amine comprising three or more amine groups, an aromatic
triamine, 1,3,5-tri(aminophenoxy)benzene, a silica cage structure
decorated with three or more amines.
6. The aerogel of claim 1, wherein the oligomer has been chemically
imidized to completion.
7. The aerogel of claim 1, wherein the oligomer has been thermally
imidized to completion.
8. The aerogel of claim 1, wherein the aerogel was obtained by
supercritical fluid extraction.
9. The aerogel claim 1, wherein the aerogel has a density of 0.077
to 0.3 g/cm.sup.3.
10. The aerogel of claim 1, wherein the aerogel has a porosity of
80 to 95%.
Description
BACKGROUND
Field
This invention generally relates to polyimide aerogels, and more
particularly to polyimide aerogels with three-dimensional
covalently bonded cross-linked structure.
DEFINITIONS
Aerogels--a unique material for providing such properties because
of their extremely low density and small pore sizes.
Anhydride--a functional derivative of a carboxylic acid in which
the OH of the carboxyl group has been replaced by --COOR.
Backbone--the series of covalently bonded atoms that together
create the continuous chain of the molecule.
Diamine--a type of polyamine with exactly two amino groups.
Dianhydride--Any compound containing two anhydride groups.
Oligomer--an oligomer consists of a limited number of monomer
units, in contrast to a polymer that, at least in principle,
consists of an unlimited number of monomers.
Polyamic acid--intermediate polymer in the production of polyimide
made by the reaction of diamine with a cyclic dianhydride.
Polyimide (sometimes abbreviated PI)--the polymeric product of
dehydration of the polyamic acid, the structure shown below being
one example.
##STR00001##
Supercritical Fluid--any substance at a temperature and pressure
above its critical point. It can diffuse through solids like a gas,
and dissolve materials like a liquid. Additionally, close to the
critical point, small changes in pressure or temperature result in
large changes in density.
Supercritical Fluid Extraction (SFE)--the process of separating one
component (the extractant) from another (the matrix) using
supercritical fluids as the extracting solvent. Extraction is
usually from a solid matrix, but can also be from liquids.
Triamine--a type of polyamine with exactly three amino groups.
DESCRIPTION OF RELATED ART
Thermosetting polyimides are commercially available as uncured
resins, stock shapes, thin sheets, laminates, and machines parts.
Thermoplastic polyimides are very often called pseudothermoplastic.
There are two general types of polyimides. One type, so-called
linear polyimides, is made by combining imides into long chains.
Aromatic heterocyclic polyimides are the other usual kind, where R'
and R'' are two carbon atoms of an aromatic ring. Examples of
polyimide films include Apical, Kapton, UPILEX, VTEC PI, Norton TH
and Kaptrex. Polyimides have been in mass production since 1955.
Typical monomers include pyromellitic dianhydride and
4,4'-oxydianiline.
Lightweight, low density structures are desired for acoustic and
thermal insulation for aerospace structures, habitats, and
astronaut equipment and aeronautic applications. Aerogel is a
manufactured material with the lowest bulk density of any known
porous solid. It is derived from a gel in which the liquid
component of the gel has been replaced with a gas. The result is an
extremely low-density solid with several properties, most notably
its effectiveness as a thermal insulator and its extremely low
density. It is nicknamed frozen smoke, solid smoke, or blue smoke
due to its translucent nature and the way light scatters in the
material; however, it feels like expanded polystyrene to the touch.
Aerogels are produced by extracting the liquid component of a gel
through supercritical drying. This allows the liquid to be slowly
drawn off without causing the solid matrix in the gel to collapse
from capillary action, as would happen with conventional
evaporation. The first aerogels were produced from silica gels.
Plain silica aerogels are brittle. Reinforcing the aerogel
structure with polymer provide improvements in strength while
maintaining low density and pore structure. Degradation of polymers
used in cross-linking tend to limit use temperatures to below
150.degree. C.
Polyimide aerogels can be fabricated from linear polyimides by
allowing a low concentration polyimide/polyamic acid solution to
gel, followed by heating to complete imidization and subsequent
supercritical fluid extraction. Polyimide aerogels prepared in this
way from, for example, oxydianiline (ODA) and pyrolimellitic
dianhydride have high surface areas, low density, low thermal
conductivity, and good ductility. However, gels shrink
substantially during supercritical fluid extraction. SFE can be
used as a sample preparation step for analytical purposes, or on a
larger scale to either strip unwanted material from a product (e.g.
decaffeination) or collect a desired product (e.g. essential oils).
Carbon dioxide (CO.sub.2) is the most used supercritical fluid,
sometimes modified by co-solvents such as ethanol or methanol.
Extraction conditions for supercritical CO.sub.2 are above the
critical temperature of 31.degree. C. and critical pressure of 74
bar. Addition of modifiers may slightly alter this. The discussion
below will mainly refer to extraction with CO.sub.2, except where
specified. Supercritical fluids are suitable as a substitute for
organic solvents in a range of industrial and laboratory processes.
Carbon dioxide and water are the most commonly used supercritical
fluids, being used for decaffeination and power generation
respectively. In addition, there is no surface tension in a
supercritical fluid, as there is no liquid/gas phase boundary. By
changing the pressure and temperature of the fluid, the properties
can be "tuned" to be more liquid or more gas like. One of the most
important properties is the solubility of material in the fluid.
Solubility in a supercritical fluid tends to increase with density
of the fluid (at constant temperature). Since density increases
with pressure, then solubility also tends to increase with
pressure. The relationship with temperature is a little more
complicated. At constant density, solubility will increase with
temperature. However, close to the critical point, the density can
drop sharply with a slight increase in temperature. Therefore,
close to the critical temperature, solubility often drops with
increasing temperature, then rises again. All supercritical fluids
are completely miscible with each other so for a mixture a single
phase can be guaranteed if the critical point of the mixture is
exceeded. The critical point of a binary mixture can be estimated
as the arithmetic mean of the critical temperatures and pressures
of the two components, T.sub.c(mix)=(mole fraction
A).times.T.sub.cA+(mole fraction B).times.T.sub.cB
For greater accuracy, the critical point can be calculated using
equations of state or group contribution methods. Other properties,
such as density, can also be calculated using equations of
state.
Aerogels are solid materials that consist of a highly porous
network of micro-sized and meso-sized pores. The pores of an
aerogel can frequently account for over 90% of the volume when the
density of the aerogel about 0.05 gram/cc. Aerogels are generally
prepared by a supercritical drying technique to remove the solvent
from a gel (a solid network that encapsulates its solvent) such
that no solvent evaporation can occur and consequently no
contraction of the gel can be brought by capillary forces at its
surface. Therefore, aerogel preparation through a sol-gel process
proceeds in 3 steps: dissolution of the solute in a solvent,
formation of the sol, formation of the gel, and solvent removal by
either supercritical drying technique or any other method that
removes solvent from the gel without causing pore collapse.
Typically, the synthesis of polyimide gels at very low solute
concentration is the first step in the preparation of polyimide
aerogels. Precursor poly(amic acids) are imidized in solution at
elevated temperatures, some polyimides will gel as the reaction
solution is quenched from the high reaction temperature to ambient
temperature. However, solution imidization at elevated temperatures
is accompanied by hydration leading to depolymerization of the
poly(amic acids) and results in a weakened gel. Such gels do not
have sufficient mechanical strength to yield low-density polyimide
aerogels. It has been reported that chemical imidization of some
poly(amic acids) at a solute concentration above 10 to 15%
(wt./wt.) produces gels probably induced by intermolecular
cross-linking. Such gels are mechanically weak and the high solute
concentrations are not feasible for producing a low-density
aerogel. (Where did this come from?)
Reinforcing of polymers and polymer matrix composites has
conventionally been done by the inclusion of particles or fibers
into the polymer matrix. The adhesion between the matrix and the
reinforcing materials has typically been weak, limiting the
performance of the reinforced system compared to its theoretical
performance. Fibers are often used with a coating which is intended
to become involved with the polymer matrix, strengthening the
interface between matrix and reinforcement. These interface
modifiers have met with varied degrees of success, with the end
result that the interface between the reinforcement and the matrix
is still a weak spot, particularly for high-temperature materials
such as polyimides.
SUMMARY
Polyimide aerogels with three-dimensional covalently bonded
cross-linked structure which are made using linear oligomeric
segments of polyimide and linked with one of the following into a
three dimensional structure: trifunctional aliphatic or aromatic
triamines; latent reactive endcaps such as nadic anhydride or
phenylethynylphenyl amine; silica or silsesquioxane cage structures
decorated with amine. Drying the gels supercritically maintains the
solid structure of the gel, creating a polyimide aerogel with
improved mechanical properties over linear polyimide aerogels.
At least one of the embodiments provides for the manufacture of
small (micro to nano-scale) particles of silica or other ceramic
materials which are covalently bonded to a polymer fragment.
Other benefits and advantages will become apparent to those skilled
in the art to which it pertains upon reading and understanding of
the following detailed specification.
DETAILED DESCRIPTION
In one embodiment, a polyimide aerogel fabricated by using tri,
tetra, or polyfunctional units in the structure to create a
three-dimensional covalently bonded network. Such cross-linked
polyimides typically have a higher glass transition temperature
above 300 to 400.degree. C. The reinforcement provided by the
three-dimensional network should improve mechanical stability, and
prevent shrinkage on supercritical fluid extraction. One example of
this embodiment uses 1,3,5-(triaminophenoxy)benzene (TAB) with
diamines and dianhydride to form the three-dimensional polyimide
according to Scheme 1. Gelation of the polyamic acid occurs rapidly
in 5-10 w/w % solutions when n=1 to 10. Subsequent heating with or
without catalysts to affect imidization followed by supercritical
fluid extraction gives three-dimensional polyamide aerogel
structures.
##STR00002##
Example 1
3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA),
3,4-oxydianiline (ODA), 1,3,5-tris-(4-aminophenoxy)benzene (TAB)
are used to make three dimensional cross-linked aerogels using
N-methyl-2-pyrrolidinone (NMP), dimethylformamide (DMF), or
dimethylacetamide (DMAc) as solvent. 1,4-diazabicyclo[2.2.2]-octane
(DABCO) is used as catalyst for imidization. Poly amic acid
solution was prepared as a 10% (w/w) solution of diamine,
dianhydride, and triamine dissolved in 20 mL DMF. BTDA (1.1761 g),
ODA (0.3654 g), and small amount of DABCO were placed into a 125 mL
glass vial, which was capped. Into a second 125 mL glass vial, TAB
(0.4860 g) was measured and the vial was capped. The vials were
placed into a glovebox, and 10 mL of dry DMF was pipetted into each
glass vial to dissolve the contents. Once all solutions were
completely dissolved, TAB solution was added to the BTDA/ODA/DABCO
solution and the new solution was mixed quickly and poured into
glass molds. Poly amic acid gels formed in less than one minute.
The molds containing poly amic acid gels were placed into an oven
at 115.degree. C. for 81/2 hours to imidize the gels. Imidized gels
were extracted into a solution of 25% acetone in DMF and soaked for
twenty-four hours. Solution was drained and replaced with 50%
acetone in DMF and soaked for twenty-fours. This was repeated at
75% acetone in DMF and 100% acetone. Polyimide gels were dried
supercritically using an Applied Separations supercritical dryer.
Acetone was replaced with liquid CO.sub.2, then temperature and
pressure were increased until CO.sub.2 became supercritical. All
solvent was removed, producing polyimide aerogels with no shrinkage
and having a density of 0.09 g/cm3 and surface areas from nitrogen
sorption spectroscopy of 400 m.sup.2/g. Solid NMR, FTIR, DSC, and
TGA confirmed that imidization was complete. Young's modulus of
compression for the aerogels was 0.6 MPa.
Example 2
BTDA (0.961 g) and ODA (0.448 g) were dissolved in 12 ml of DMF
solvent was added to dissolve the solutes. In tube B, TAB (0.199 g)
was dissolved in 2.24 ml DMF. The two solutions were quickly mixed
at room temperature after dissolving. The mixture was poured into a
mold made by cutting the end off of a twenty ml disposable syringe.
Polyamic acid gel was formed in 30 seconds. The gel was extracted
into a solvent solution that contained 4 ml of acetic anhydride,
3.4 ml of pyridine, and 72.60 ml of DMF. After aging in this
solvent solution overnight, the polyamic acid gel was imidized in
the oven at 115.degree. C. for 8.5 hours. The polyimide wet-gel was
first washed in 100% DMF, then 75%/25% DMF/acetone, followed by
50%/50% DMF/acetone, then 25%/75% DMF/acetone, and finally 100%
acetone in twenty four hour increments. After the final solvent
exchange, the polyimide wet-gel was supercritical dried in the
CO.sub.2 supercritical dryer. The resulting aerogel had a density
of 0.077 g/cm.sup.3. Young's modulus of compression for the aerogel
was 0.6 MPa.
Example 3
For a polyimide aerogel with oligomer length between TAB
cross-links of n=3, biphenyl 3,4,3',4'tetracarboxylic dianhydride
(BPDA) (0.82 g) and Bis(aminophenyl)xylylene (BAX) (0.60 g) were
dissolved in 12 ml of NMP solvent. TAB (0.180 g) was dissolved in
1.13 ml NMP. The two solutions were quickly mixed at room
temperature after dissolving. The mixture was poured into a mold
made by cutting the end off of a twenty ml disposable syringe.
Polyamic acid gel was formed in 20 seconds. The gel was extracted
into a solvent solution that contained 4 ml of acetic anhydride,
3.4 ml of pyridine, and 72.60 ml of NMP. After aging in this
solvent solution overnight, the polyamic acid gel was imidized in
the oven at 170.degree. C. for 4 hours. The polyimide wet-gel was
first washed in 100% NMP, then 75%/25% NMP/acetone, followed by
50%/50% NMP/acetone, then 25%/75% NMP/acetone, and finally 100%
acetone in twenty four hour increments. After the final solvent
exchange, the polyimide wet-gel was supercritical dried in the
CO.sub.2 supercritical dryer. The resulting aerogel had a density
of 0.8 g/cm.sup.3.
Example 4
For a polyimide aerogel with oligomers length of n=5 between TAB
cross-links, BTDA (0.564 g) and BAX (0.421 g) were dissolved in 8
ml of DMF solvent in tube A. In tube B, TAB (0.078 g) was dissolved
in 1.49 ml DMF. The two solutions were quickly mixed at room
temperature after dissolving. The mixture was poured into a mold
made by cutting the end off of a twenty ml disposable syringe.
Polyamic acid gel was formed in 43 seconds. The gel was extracted
into a solvent solution that contained 4 ml of acetic anhydride,
3.4 ml of pyridine, and 72.60 ml of DMF. After aging in this
solvent solution overnight, the polyamic acid gel was imidized in
the oven at 115.degree. C. for 8.5 hours. The polyimide wet-gel was
first washed in 100% DMF, then 75%/25% DMF/acetone, followed by
50%/50% DMF/acetone, then 25%/75% DMF/acetone, and finally 100%
acetone in twenty four hour increments. After the final solvent
exchange, the polyimide wet-gel was supercritical dried in the
CO.sub.2 supercritical dryer. The resulting aerogel had a density
of 0.9 g/cm.sup.3.
In another embodiment, cross-linking of the polyimide can be
carried out by capping long chain oligomers with latent reactive
endcaps such as nadic anhydride or phenylethynlyphenylamine, and
after supercritical fluid extraction, the cross-linking is carried
out on a post cure of the dried gels. In one embodiment, nadic
anhydride is used as the endcaps as shown in Scheme 2. Nadic capped
polyimide oligomers of n=4 to 20 gel within 15 to 50 minutes.
Supercritical fluid extraction and subsequent heating of the
aerogel to 300-315.degree. C. causes the nadic groups to cross-link
as shown, giving a stable network structure.
##STR00003##
Example 5
For n=10 oligomer from Scheme 2, BPDA (4.0940 g) and BAX (4.4753 g)
were heated and stirred in 20 ml N-methyl-2-pyrrolidinone (NMP) to
dissolve. In the meantime, nadic anhydride (0.4569 g) was dissolved
in 5 ml NMP. When the mixture of BPDA and BAX was cooled down, the
solutions combined in one clean and dried 50 ml volumetric flask,
and the volumetric flask was filled to 50 ml with NMP and shaken to
mix the solution. The solution in the volumetric was combined with
acetic anhydride (10 ml) and pyridine (5 ml) and poured into molds.
Gelation occurred within 30 minutes. Gels were extracted into clean
NMP and heated in oven at 110.degree. C. for 8.5 hours. Afterwards,
gel solvent is exchanged gradually into acetone as previously
described. Supercritical drying yields aerogels with density of
0.12 g/cm.sup.3. Subsequent heating of aerogel to 300.degree. C.
causes nadic endcaps to cross-link as shown in Scheme 2.
In another embodiment, polyimide or polyamic acid can be capped
with trialkoxy silanes which can be co-reacted with small amounts
of tetraalkoxy silanes to form regions of covalently bonded silica
crosslinks as shown in Scheme 3. In this case, n=1 to 10 polyimide
oligomers combined with pre-hydrolyzed tetraethyl orthosilicate
(TEOS) and base catalyst.
##STR00004##
In another embodiment, similar to that shown in Scheme 3, TEOS can
be replaced with polysilsesquioxane cages (POSS) decorated with
surface amines to co-react with anhydride capped polyimide/polyamic
acid. In this case, anhydride capped polyimide or polyamic acid
oligomers of n=1 to 10 can be reacted with triamino- or
tetraamino-POSS at room temperature. Gelation occurs in 5 to 50
minutes, followed by heating to 110.degree. C. with catalyst or
180.degree. C. without catalyst to imidize.
Notwithstanding that the numerical ranges and parameters setting
forth the broad scope of the invention are approximations, the
numerical values set forth in the specific examples are reported as
precisely as possible. Any numerical value, however, inherently
contain certain errors necessarily resulting from the standard
deviation found in their respective testing measurements.
The embodiments have been described, hereinabove. It will be
apparent to those skilled in the art that the above methods and
apparatuses may incorporate changes and modifications without
departing from the general scope of this invention. It is intended
to include all such modifications and alterations insofar as they
come within the scope of the appended claims or the equivalents
thereof.
* * * * *